Are all lawn grubs the same insect?

A brown patch showed up in the lawn a few weeks ago and kept spreading, even though you have been watering. Then one morning you find the edges of it torn up, as if something had been digging all night. You grab a corner of the dead grass and it lifts like a loose rug, and underneath are fat, curled, cream colored grubs.

It is tempting to call them all "the grubs" and go buy something labeled grub killer. But lawn grubs are not one insect. The name covers the young, soil dwelling stage of many different beetles, and a few other creatures that just look similar. Japanese beetles, European chafers, May and June beetles, masked chafers and Asiatic garden beetles all start life as white grubs in the soil, and they do not all behave the same way.

At a glance they are remarkably alike. Most are plump, C shaped and milky white to grayish white, with a brown or tan head and three pairs of small legs near the front. They live in the top one to three inches of soil and eat grass roots. As the roots are cut away, the grass can no longer take up water, so it thins, yellows and wilts in a way that looks just like drought, even when you water faithfully. Eventually the roots are gone completely, which is why the dead turf peels back so easily.

Here is the frustrating part: you cannot tell the beetle species apart by color or size. The reliable feature is on the underside of the last segment of the grub's body, a patch of tiny hairs and spines called the raster. Its pattern differs from species to species, and seeing it takes a 10x to 20x hand lens. The Japanese beetle grub, for example, has a V shaped arrangement there. If you want certainty, your local extension service can identify a sample.

Life cycles differ too, and that changes your planning. Japanese beetles, European chafers and masked chafers complete their life cycle in a single year. May and June beetles can spend two or three years in the soil before they become adults.

Some grubs are not beetle grubs at all. If the larva in the grass has no legs and is cream colored, feeding at the base of the grass stems, it is likely a billbug larva. If it is gray, legless and looks more like a maggot, especially in damp soil, it is probably a leatherjacket, the larva of a crane fly. Both need different handling from white grubs.

To check your lawn, dig rather than soak. With a flat spade, cut a square of sod about a foot on each side and two to three inches deep, peel it back, shake the soil from the roots, and count the grubs. Do this in a few places around the damaged area. A soap flush, where you pour soapy water on the lawn, is often recommended online, but it works for surface dwellers like armyworms, cutworms and mole crickets. White grubs live in the soil and will not come up for it.

How many is too many depends on the lawn. Unwatered or stressed grass can show serious damage with just three to five grubs per square foot. A well watered lawn with healthy roots may carry twenty or more without browning.

Often the most visible damage is not from the grubs at all. Skunks, raccoons, armadillos, moles and birds love a meal of grubs and tear up turf to find them, and many people notice the digging before they ever see wilting. Dealing with the grubs is what stops the digging.

A healthy lawn tolerates grubs much better, so the basics matter. Keep thatch, the layer of dead material between the soil and the grass blades, under about half an inch. Water deeply and less often to build deep roots, and avoid soggy soil, which attracts egg laying beetles. Mow at the height recommended for your grass type.

This is where knowing your grub pays off. Milky spore, a popular biological product, works only against Japanese beetle grubs, takes several years to build up in the soil, and does nothing against chafers or May and June beetles. It also fails in cooler northern soils. Beneficial nematodes, microscopic worms that attack grubs, work only above about 60°F soil temperature, should go down in the evening away from strong sunlight, and need moist soil before and after application. Different nematode species target different pests, so match the product to the grub.

Timing matters as much as the product. The big grubs you find in spring are mature and about to pupate. They eat little, resist most treatments and are hard to kill. Treatment works best in early to midsummer, when the new generation hatches and the grubs are small. If you do use an insecticide, follow the label exactly, and water it in afterward as the label directs so it reaches the root zone rather than sitting on top of the thatch.

Your local extension service is the best first call if you are not sure what you are dealing with. A sample under magnification settles the species question quickly, and that answer decides which treatments are worth your money.

Consider a lawn care professional for large areas, repeat damage year after year, or when the digging animals themselves have become the bigger problem.

Rules also vary by place. Some beetles and grubs are regulated or watched more closely in certain regions and countries, so check with your local agriculture or plant health authority if you are outside the United States or suspect something unusual.

So no, lawn grubs are not all the same insect, and that is exactly why a one size fits all grub killer so often disappoints. Dig a sample, figure out whose grub it is, and treat when they are small.